Transients of Carrier Recombination and Diffusion in Highly Excited GaN Studied by Photoluminescence and Four-Wave Mixing Techniques

Abstract

Time-resolved photoluminescence and four-wave mixing techniques have been combined for studies of carrier relaxation dynamics in a highly photoexcited GaN epilayer. For a moderate excitation density below 1 mJ/cm 2 , carrier recombination was due to free carrier capture by deep traps. The characteristic time of carrier capture, τ e = 550 ps, was measured under deep trap saturation regime. The ambipolar diffusion coefficient for free carriers, D = 1.7 cm 2 /s, was estimated from the analysis of the transients of the light-induced gratings of various periods. A complete saturation of the four-wave mixing efficiency was observed for the excitation energy density exceeding 1.5 mJ/cm 2 . The latter saturation effect was shown to be related to electron-hole plasma degeneration, which results in a significant enhancement of carrier recombination rate due to onset of stimulated emission.

Contribution of dislocations to carrier recombination and transport in highly excited ELO and HVPE GaN layers

“…3b). In the ELO layer, however, due to a larger dislocation density, we observed a gradual transfer from linear to bimolecular recombination only at higher excitation, with subsequent saturation of EC at reaching the threshold of stimulated recombination [2][3][4].…”

“…The previous works have demonstrated the applicability of time-resolved four-wave mixing (TR FWM) for investigation of carrier recombination and transport features in differently grown GaN heterostructures [1,2]: a novel possibility to determine the threshold of stimulated recombination [2,3], carrier localization dependent diffusion coefficient in InGaN alloys, decrease of the alloy structural quality at higher In content, evidenced by the decreasing carrier lifetime [3]. Complementary studies by using photoluminescence (PL) and time-resolved PL have confirmed a high sensitivity and advantages of FWM for studies of defect-density and excitation dependent carrier recombination peculiarities and transport in GaN heterostructures [2,[4][5][6]. However, the previous studies by TR FWM have been performed at 300 K.…”

Carrier dynamics in Fe‐doped GaN epilayers

“…2, two different regions are visible in the plots: at low excitation (< 0.4 mJ/cm 2 ), η increases with pump as η = I 0 2 and then a saturation of the diffraction efficiency takes place. It has been established that the mentioned regions are caused by two different physical processes: first, the band-to-band carrier generation increases the diffraction signal, but then the stimulated recombination evolves as the threshold carrier concentration is reached [18]. The saturation in the undoped REF1 is much stronger pronounced than in the doped samples MD1 and MD2, which we attribute to a lower efficiency of the stimulated recombination in the Fe-doped GaN.…”

Time-Resolved Transient Grating Spectroscopy for Studies of Nonequilibrium Carrier Dynamics in Wide Band-Gap Semiconductors

“…Dynamic holography provides optical configurations to create and monitor the spatial refractive index modulation n(x, t) = n 0 + ∆n(t) cos (Kx) as well relationship between the diffraction efficiency of a grating η = (π∆nd/λ) 2 , while the nonlinear optics gives quantitative ratios between the modulation mechanisms of optical properties and the modulation amplitude ∆n. In the given case, the refractive index change at 1064 nm (well below the band gap) is determined by free carrier density N according to Drude-Lorentz model: ∆n = −n eh ∆N , where the coefficient n eh is the refractive index change by one electron-hole pair n eh [3] and equals to ≈ 1 × 10 −21 cm −3 for GaN at the wavelength of the probe beam. This coefficient together with the thickness of excited region d determines the sensitivity of the FWM technique, i.e.…”

“…Direct relationship between nonresonant, light induced refractive index modulation ∆n(x, t) by nonequilibrium carriers ∆N (x, t) and holographic approach for monitoring dynamics of ∆n allowed development of innovative holographic measurement technique -the time-resolved (TR) four-wave mixing (FWM) on free carrier gratings. Recently TR FWM under picosecond excitation was applied for study of competition between nonradiative recombination at dislocations and radiative interband processes in GaN layers with varying dislocation density [1], determination of bimolecular recombination coefficient and its temperature dependence [2] in hydride vapor phase epitaxy grown (HVPE-grown) free standing films, control of stimulated emission threshold in lasing compounds [3], carrier localization effects in InGaN alloys by direct measurements of In-content dependent diffusion coefficient values and carrier lifetimes values [4], recombination activity of iron-related traps and dislocations in highly resistive GaN layers [5]. In epitaxial SiC, origin of fast decay transients of bipolar nonequilibrium carrier plasma was analysed [6].…”